1. Field of the Invention
The present invention relates to an apparatus and a method for printing images; the invention especially concerns the printer configuration. The invention is particularly suitable for ink-jet printing. The invention is particularly suitable for duplex printing and may also be applied to simplex printing.
2. Background of the Invention
Whereas in simplex printing an image is printed on only a single side of a receiving substrate such as a sheet of paper, in duplex printing images are printed on both sides. When applying liquid ink to the receiving substrate in order to print the image, the wet receiving substrate first has to dry before it can be processed further; e.g. when printing on paper sheets, the printed sheet must be dry before the next sheet can be stacked on top of it.
U.S. Pat. No. 4,469,026 discloses a printer having a sheet fed and drum transport assembly. Ink is applied to a sheet while it is transported by the drum. Subsequently, the receiving substrate is detached from the drum and conveyed by a vacuum belt past a dryer.
U.S. Pat. No. 5,712,672 discloses a printer wherein sheets are transported by means of a vacuum belt past an ink-jet printhead and through a microwave dryer.
Patent application WO 99/11 551 discloses a printer wherein sheets are transported by a vacuum drum. A simplex printer has one vacuum drum, while a duplex printer uses two counter-rotating drums. In a duplex printer, a first image is printed on one side of a paper sheet while the sheet is on the first drum; then the paper is fed to the second drum so that the first printed image contacts the second drum, and a second image is printed on the opposite side of the paper. The printer can also be used to print on a continuous web instead of on separate sheets.
U.S. Pat. No. 4,609,517 discloses a printer having a device for flattening curled sheets subsequent to printing and at least partial drying thereof. The sheets are transported by a belt that has a straight portion along which a print module and a drying module are located.
U.S. Pat. No. 5,623,288 discloses several embodiments of a printer for making enlarged prints on a continuous web of receiving substrate. In a specific embodiment, the receiving substrate is passed around a first drive roller while ink is applied to the first side of the receiving substrate The receiving substrate is then dried along a straight portion of its path. Subsequently, the receiving substrate is passed around a second drive roller while ink is applied to the second side of the receiving substrate, opposite to the first side. Then, the receiving substrate is dried again, along another straight portion of its path.
U.S. Pat. No. 5,966,145 discloses a printer for textile printing on a cloth. A thin endless metallic belt transports the cloth past two printing units and a drying unit in-between that are all located along a straight portion of the belt. The cloth is separated from the belt and dried again by a post-drying unit located at another straight portion of the cloth path.
U.S. Pat. No. 4,566,014 discloses a sheet printer wherein the gap between successive sheets is adjusted for optimal drying of the sheets. The printer has a printing unit that is located along a straight portion of the sheet path. After printing, the sheet is dried in a drying unit along a straight portion of a first belt, a portion of a drum and a straight portion of a second belt. Optionally, the printed sheet may be re-fed to the printing unit and the drying unit for duplex printing.
A disadvantage of the printers described above is that they are not compact. This is especially the case in high speed printing, because at high speed the processing operations in the printer, such as drying the receiving substrate, require quite some space.
It is an object of the present invention to provide a printing apparatus that has a compact configuration.
It is an object of the present invention to provide a method for producing an image on a receiving substrate by means of a printer, so that the printer is compact.
A xe2x80x9creceiving substratexe2x80x9d may be a separate sheet or it may be a continuous web; it may be made of paper, of polyethylene coated paper, of plastic, of white poly(ethylene terephtalate), of another material as known in the art; it may be a laminate of two or more materials; it may comprise one or more special layers such as an image-receiving layer; it may be transparent or opaque. A receiving substrate has two sides opposite to each other; in simplex printing an image is printed on only a single side, in duplex printing images are printed on both sides.
xe2x80x9cLiquid inkxe2x80x9d is ink that is in the liquid state of aggregation when it is applied to the receiving substrate. Thus, liquid ink includes e.g. the following types of ink, known in the art: water based ink, oil based ink, solvent based ink, hot melt ink. Whereas the first three types of ink are liquid at room temperature, hot melt ink is solid at room temperature and is applied at a temperature higher than room temperature.
A xe2x80x9ctouch-dryxe2x80x9d receiving substrate is a receiving substrate, or a portion thereof, that is substantially dry so that, after printing, mutual contact of the fresh prints is possible without causing smudges. Usually, after printing, separate sheets are stacked on top of each other, while a continuous web may be wound onto a roll or cut into sheets that are stacked, so that portions of the printed web contact each other.
A xe2x80x9cdrying sectionxe2x80x9d is a section, or portion, of the apparatus wherein the receiving substrate, still containing wet ink originating from the ink application, is subjected to a drying process so that it becomes touch-dry. The drying process may be different, depending on the type of ink; e.g.:
for water based ink, the drying process involves absorption and penetration of the ink in the receiving substrate and evaporation of water from the ink;
for oil based ink, the drying process involves absorption and penetration;
for solvent based ink, the drying process primarily involves evaporation;
for hot melt ink, the drying process involves solidification of the ink.
The drying process can occur in a xe2x80x98passivexe2x80x99 way, or in an xe2x80x98activexe2x80x99 way by using drying means in order to accelerate the drying process. For water based ink, for instance, natural air drying is a xe2x80x98passivexe2x80x99 way of drying, whereas xe2x80x98activexe2x80x99 drying involves using drying means such as infrared lamps, microwave energy applicators, hot air applicators, or other drying means as known in the art; a combination of passive and active drying may also be used.
xe2x80x9cDrying meansxe2x80x9d are discussed in the definition of a xe2x80x9cdrying sectionxe2x80x9d above.
A xe2x80x9cconvex curvexe2x80x9d along which a printed receiving substrate is transported is a curve that has its centres of curvature xe2x80x98CCxe2x80x99 further away from the printed side xe2x80x98PRxe2x80x99 of the receiving substrate than from the other side xe2x80x98OSxe2x80x99 of the receiving substrate; i.e. along a straight line starting at a centre of curvature CC of the curve and intersecting the receiving substrate, the order wherein the sides are encountered is: CC, OS, PR. The printed side PR of the receiving substrate is that side which was printed last; it may still contain wet ink. FIG. 1 shows a convex curve 31 (ink is applied last by ink application means 11) and a concave, i.e. non-convex curve 59 (ink is applied last by ink application means 21). FIG. 1 is further discussed below. A convex curve may be a circular curve or a non-circular curve. A circular curve has one centre of curvature, viz. the centre of the circle of which the curve forms a part. For a non-circular curve, each point P of the curve has a corresponding centre of curvature CC which is defined as the limiting position of the point of intersection of the normals at P and at a neighbouring point Q, as Q is made to approach P along the curve (see e.g. xe2x80x9cMarks"" Standard Handbook for Mechanical Engineersxe2x80x9d, Baumeister et. al, ISBN 0-07-004123-7, McGraw-Hill, eighth edition, page 2-45). A xe2x80x9cconcave curvexe2x80x9d along which a printed receiving substrate is transported is a curve that has its centres of curvature CC closer to the printed side PR of the receiving substrate than to the other side OS of the receiving substrate.
The xe2x80x9cangle covered by a curvexe2x80x9d is the angle between the normals at the endpoints of the curve. For a circular curve, this angle can also be calculated as: the length of the curve, divided by the radius of the circle of which the curve forms a portion, multiplied by 180xc2x0/pi to convert the angle from radians to degrees.
A xe2x80x9cconvex arcxe2x80x9d means in this document a small convex curve, covering an angle of e.g. 5xc2x0 or less.
The above mentioned objects are realised by a printing apparatus having the specific features defined in independent claims 1 and 2 and by a method having the steps defined in independent claims 11 and 12. Specific features for preferred embodiments of the invention are set out in the dependent claims.
A printing apparatusxe2x80x94also called a printerxe2x80x94as claimed provides a compact configuration, especially for high speed printers where the processing operations, such as applying the ink, drying the receiving substrate, transporting it, require quite some space since the operations need a given time. The path of the receiving substrate in a printer in accordance with the invention is such that, for a given floor space and a given height of the printer, a large path length is available for the required processing operations.
FIG. 1 shows a first embodiment of a duplex printer 10 in accordance with the invention. A receiving substrate 20 is taken from an input stack 61 of sheets and conveyed by input rollers 62 and roller 16 to belt 14. Belt 14 is guided by pulleys 13 and moves clockwise, in the sense of arrow A1; it first transports the receiving substrate 20 past ink application means 11 that applies liquid ink to one side of the receiving substrate 20 and so prints a first image. The receiving substrate is then transported by belt 14 through a drying section 12 where the receiving substrate becomes touch-dry. Subsequently the receiving substrate 20 is transferred to a second belt 24 in take-over section 25, so that the side of the receiving substrate that contains the first image contacts belt 24. Belt 24 is guided by pulleys 23 and moves counterclockwise, in the sense of arrow A2; i.e. belts 14 and 24 are counterrotating. Belt 24 transports the receiving substrate past ink application means 21 that prints a second image on the other side of the receiving substrate, so that both sides of the receiving substrate now contain a printed image. The receiving substrate is further transported by belt 24 through drying section 12 in order to become touch-dry. Finally, the receiving substrate is transported by roller 26 and conveyed by output rollers 63 to output stack 64.
In the embodiment shown in FIG. 1, belt 14 transports receiving substrate 20 along a substantially polygonal path 30,31,32,33,34,35,36, abbreviated as 30-36, past ink application means 11 and through drying section 12. Path 30-36 comprises, in the sense of arrow A1:
a substantially straight portion 30, consisting of substantially straight portions 30a up to position P1, 30b between positions P1 and P2 and 30c beyond position P2;
a convex curve 31;
a substantially straight portion 32, consisting of substantially straight portions 32a up to position P3 and 32b beyond position P3;
a convex curve 33;
a substantially straight portion 34, consisting of substantially straight portions 34a up to position P4 and 34b beyond position P4;
a convex curve 35;
a substantially straight portion 36, consisting of substantially straight portions 36a up to position P6 and 36b beyond position P6.
Positions P1-P7 along path 30-36 are located as follows: P1 and P2 at the ink application means 11, P3 and P4 in the drying section 12, P5, P6 and P7 in the take-over section 25.
xe2x80x9cDownstreamxe2x80x9d is the term that is used to indicate the location of P1-P7 relative to each other, in the transport direction of the receiving substrate, i.e. with respect to the sense of arrow A1 in FIG. 1: position P7 is located downstream position P6, P6 is downstream P5, . . . , P2 is downstream P1. Conversely, P1 is xe2x80x9cupstreamxe2x80x9d P2, etc.
The path 30-36 of the receiving substrate 20 in the embodiment shown in FIG. 1 is substantially polygonal: it is a polygon with rounded edges. The rounded edges, i.e. convex curves 31, 33, 35, may be realised by guiding belt 14 around pulleys 13. A substantially straight portion such as portions 30, 32, 34, 36 may be realised by tightening belt 14 between two pulleys 13. An advantage of a substantially polygonal path is that a large path length is available for the required processing operations, so that the printer may be compact. In fact, for a given floor space and printer height, a circular drum as disclosed in WO 99/11 551 only provides the circumference of the circle as available path length for processing operations. A substantially polygonal path, on the other hand, provides the perimeter of the rounded polygon as available path length, which may be considerably larger than the circle circumference of a circular drum if the polygon is a circumscribed polygon of the circle. The circumscribed polygon may fit within the same given dimensions of floor space width, floor space length and printer height as the corresponding circular drum, while the circumscribed polygon provides a larger available path length than the drum. In a preferred embodiment, the path is substantially rectangular. A substantially rectangular path that has dimensions that exactly fit within the given width, length and height dimensions theoretically provides the maximum available path length for the given dimensions.
However, the available path length is not the only important issue; for duplex printing, two paths and the means to carry out the required processing operations have to fit within the given dimensions. Path 50-59 in FIG. 1 includes a portion 50-58 of a substantially polygonal path, realised by belt 24 moving in the sense of arrow A2, and it includes a semicircle 59, realised by roller 26. A portion of a polygonal path, such as portion 50-58, also provides the advantage of a large available path length (remark: the even reference numbers 50, 52, 54, 56 and 58 indicate substantially straight portions, while the odd reference signs 51, 53, 55 and 57 indicate convex curves). Moreover, as is clear from FIG. 1, the combination of paths 30-36 and 50-59 through respectively the upper half and the lower half of duplex printer 10 provides a compact arrangement and a large available path length for the processing operations required to print an image on both sides of the receiving substrate 20.
FIGS. 3 to 7 show other embodiments in accordance with the invention. In FIG. 3, belts 14 and 24 have a substantially triangular shape, while in FIGS. 4 to 7 the path of the receiving substrate 20 includes portions of rounded rectangles.
Another advantage of the invention is flexibility: it is easy, e.g. during the design phase of the printer, to adapt the path of the receiving substrate. In fact, a vertex of the polygon portion associated with a portion of a substantially polygonal path may easily be displaced, e.g. by displacing (see FIG. 1) a pulley 13 that guides belt 14. Moreover, because of this flexibility, the printing apparatus may be made even more compact since the path can easily be adapted to make room for a specific portion of the printing apparatus.
Yet another advantage of the invention is that the processing operations may be carried out along substantially straight portions of the path. This simplifies construction of the means that are used to carry out these operations; it is also particularly advantageous for ink application, as is discussed in detail below.
The advantages discussed above are provided by a path of the receiving substrate that has a path section that includes a number of substantially straight portions and curves between these substantially straight portions, such as substantially straight portions 30, 32, 34, 36 and curves 31, 33, 35 in path 30-36 in FIG. 1. Preferably, the curves are convex curves since the printing apparatus applies liquid ink, so that the printed side of a receiving substrate preferably does not touch any part of the apparatus before it is touch-dry. However, the path of the receiving substrate may also include concave curves; see e.g. FIG. 6 wherein the path between positions P2 and P3 includes convex curve 31 and concave curve 42, both located between substantially straight portion 30c and substantially straight portion 32a (remark: concave curve 42 is further discussed hereafter, at the discussion of transfer from one belt to another one).
In order to provide the advantages mentioned above, the length of the substantially straight portions is preferably larger than the length of the convex curves. Therefore, in a preferred embodiment, with LSTRAIGHT the sum of the lengths of the substantially straight portions in the concerned path section and with LCURVES the sum of the lengths of the convex curves in the concerned path section, LSTRAIGHT greater than k*LCURVES with k greater than 1, preferably k greater than 2, more preferably k greater than 3 and most preferably k greater than 4. Preferably, the concerned path section comprises at least one large convex curve, more preferably at least two large convex curves, wherein a large convex curve is a curve covering an angle not smaller than 10xc2x0, preferably larger than 20xc2x0, more preferably larger than 30xc2x0, still more preferably larger than 45xc2x0 and most preferably larger than 60xc2x0.
In a preferred embodiment of the invention, the path of the receiving substrate 20 has a path section that is delimited by a first position at the ink application means 11 and by a second position downstream the first position, so that the path section includes a convex curve between two substantially straight portions. For instance path 30-36 in FIG. 1 has a path section 30b, 30c, 31, 32a that is delimited by positions P1 and P3 downstream P1 and this path section includes a first substantially straight portion 30b and 30c, a second substantially straight portion 32a, and a convex curve 31 between the first and second substantially straight portions (remark: the meaning of xe2x80x9candxe2x80x9d in substantially straight portion 30b and 30c is that the portion is composed of adjoining sub-portions 30b and 30c). Some other examples of this preferred embodiment are: in FIG. 3: path section 30c, 31, 32 between position P2 at ink application means 11 and position P3 which is in the drying section (not shown in FIG. 3); in FIG. 4: path section 30b, 30c, 31,32a between position P1 at ink application means 11 and position P3 in the drying section (not shown in FIG. 4).
In another preferred embodiment of the invention, the path of the receiving substrate 20 has a path section that is delimited by a first position at the ink application means 11 and by a second position downstream the first position, so that the path section includes three substantially straight portions and two convex curves, so that each convex curve is between two substantially straight portions. Some examples are: in FIG. 1: path section 30c, 31, 32a, 32b, 33, 34a between P2 and P4 which is in the drying section; in FIG. 5: path section 30b, 30c, 31, 32a, 32b, 33, 34a between P1 and P4 which is in the drying section (not shown in FIG. 5).
In yet another preferred embodiment of the invention, the path of the receiving substrate 20 has a path section that is delimited by a first position at the ink application means 11 and by a second position downstream the first position, so that the path section includes four substantially straight portions and three convex curves, so that each convex curve is between two substantially straight portions. Some examples are: in FIG. 1: path section 30c, 31, 32a, 32b, 33, 34a, 34b, 35, 36a, 36b between P2 and P7 which is in take-over section 25; in FIG. 6: path section 30c, 31, 41, 42, 32a, 32b, 33, 34a, 34b, 35, 36a, 36b between P2 and P7 which is in take-over section 25.
In still another preferred embodiment of the invention, the path of the receiving substrate 20 has a path section that is delimited by a first position that is in the drying section instead of at the ink application means, and by a second position downstream the first position, so that the path section includes either two substantially straight portions and a single convex curve, or three substantially straight portions and two convex curves, or four substantially straight portions and three convex curves, so that each convex curve is between two substantially straight portions. An example is, in FIG. 1, path section 32b, 33, 34a, 34b, 35, 36a between P3 in drying section 12 and P6 in take-over section 25.
In the embodiments discussed above, the second point that delimits the path section may be in the drying section; it may be in the take-over section (for a duplex printer); it may be both in the drying section and in the take-over section (for a duplex printer).
The path section may also have more convex curves than illustrated by the examples discussed above.
Preferably, the receiving substrate 20 is transported along its path through the drying section by means of a belt, most preferably by a vacuum belt. In a preferred embodiment of the invention, the belt through the drying section is an endless belt that is guided by at least two pulleys, preferably by at least three pulleys, most preferably by at least four pulleys (see also FIGS. 1 to 7).
The invention may be applied to duplex printers. FIGS. 1 to 5 all diagrammatically show a duplex printer. In a duplex printer, either the first path for printing a first side of the receiving substrate (path 30-36 in FIG. 1) or the second path for printing the second side, opposite to the first side, of the receiving substrate (path 50-59 in FIG. 1), or, which is preferred, both the first and the second path are in accordance with the invention.
The invention may also be applied to simplex printers. In FIG. 1, a diagrammatic side view of a simplex printer may be obtained by deleting the upper half of the printer and by directly conveying the receiving substrate 20, for instance by an extra roller, from input rollers 62 to ink application means 21. FIG. 7 shows a simplex printer that can also be used for duplex printing.
In a method in accordance with the invention; a receiving substrate, that contains an image printed with liquid ink, is partly dried, is then transported along a convex curve, such as curve 33 in FIG. 1, and is subsequently further dried.
Preferred embodiments of a method in accordance with the invention may include features of a printing apparatusxe2x80x94as claimed or as described above or belowxe2x80x94in accordance with the invention, and vice versa. For instance, a method in accordance with the invention may include the steps of imagewise applying liquid ink to a first side of the receiving substrate, and subsequently transporting the receiving substrate along three substantially straight portions and two convex curves, so that each convex curve is between two substantially straight portions.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings